1. Field of the Invention
The present invention relates to a photomask employed for a lithography process in manufacturing a semiconductor device, and to a method of manufacturing the photomask.
2. Description of the Prior Art
Generally, in the lithography process, a photomask with a particular transfer pattern formed of regions transparent to light and regions opaque to block the light is employed. The photomask is projected through a lens system onto a processed substrate having a photosensitive material layer, in order to transfer the pattern.
A sectional view of a prior art photomask is shown in (a) of FIG. 21. On the surface of a transparent substrate 1 of glass or the like, light-blocking regions 2 of Cr, MoSi or the like are formed. Because of the light-blocking regions 2, a transfer pattern is defined.
In the projected image of the photomask manufactured as mentioned above, as shown in a distribution diagram of amplitude in (b) of FIG. 21, the light transmitted by the transparent substrate 1 prevails even in the light-blocking regions 2 because of the diffraction. Since the practical light intensity is obtained as the amplitude raised to the second power, the prevalence of the light in the light-blocking regions 2 is observed, as shown in a distribution diagram of the intensity of light in (c) of FIG. 21. Thus, the resolution on the pattern transference is reduced, and there is difficulty in transferring a minute pattern with high accuracy.
As a method of preventing the reduction of the resolution caused by the diffraction, Japanese Examined Patent Publication No. 50811/1987 discloses a phase shift technique. According to this method, when transparent portions T1, T2 and so forth and light-blocking portions S1, S2, S3 and so forth of a photomask are alternately arranged as shown in (a) of FIG. 22, phase regions 3 are formed in every other one of the transparent portions T1, T2 and so forth along the direction of arrays of them. For example, in the transparent portion T2, one of the phase region 3 is formed on the transparent substrate 1 between the adjacent light-blocking regions 2. The phase region 3 has a thickness which makes a phase difference of 180.degree. between the lights transmitted and not transmitted by the phase region 3.
Thus, the lights prevailing in the light-blocking portions after transmitted by the adjacent transparent portions, as shown in (b) and (c) of FIG. 22, cancel with each other because of the interference. For example, the lights transmitted by the transparent portions T1, T2 and prevailing in the light-blocking portions S2 interfere to cancel with each other in the light-blocking portion S2. As a result, the resolution of the photomask is improved.
In FIG. 23, (a) shows a plan view of another embodiment of the photomask employing a structure shown in (a) of FIG. 22; wherein there are three of the light-blocking regions 2 of the same dimensions, disposed in parallel at the same interval. In FIG. 23, (b) shows a distribution diagram of the amplitude of the light transmitted by the area along the line I--I of FIG. 23, (a), while (c) shows a distribution diagram of the intensity of the light transmitted by the area along the line I--I; and (d) shows a distribution diagram of the amplitude of the light transmitted by the area along the line II--II of FIG. 23, (a), while (e) shows a distribution diagram of the intensity of the light transmitted by the area along the line II--II.
As will be recognized in (a) of FIG. 23, the photomask employing the above-mentioned structure is provided with a part 3a which is in the peripheral portion of the phase region 3 and is contiguous to the transparent substrate 1. As shown in (b) and (d) of FIG. 23, the light transmitted by the phase region 3 and the light transmitted by the transparent substrate 1 are out of phase by 180.degree., and in the boundary part 3a both the transmitted lights cancel with each other because of the interference. Consequently, as shown in (c) and (e) of FIG. 23, the intensity of light in the boundary 3a declines to naught.
As a result, when a photoresist layer on a processed substrate is developed with a mask pattern of the above-mentioned photomask transferred, if a photoresist layer 5 on the processed substrate 4 is of positive-type as shown in FIG. 24, the resist is left in an area encircled by a dash-dot line 6 in FIG. 24 from which the resist should have been removed. Contrarily, if the photoresist layer 5 is of negative-type, the resist is removed from an area in which the resist should have been left. The mask pattern practically employed in manufacturing a semiconductor device has the light-blocking regions 2 which are generally configured like isolated islands as shown in (a) of FIG. 23, and hence, there arises the problem that the phase region 3 as mentioned above can not be provided in some case or other.